Motion control system for direct current motors, particularly in sewing machine uses

ABSTRACT

A motion control system for the direct current motor of a sewing machine wherein an external brake applies a retarding force to the motor and the sewing machine needle mechanism once the motor has reached a predetermined essentially stable level. Braking is controlled by monitoring the emf or current of the motor when it is in a dynamic braking mode and by initiating stabilized speed control of the motor when the emf or current obtains a predetermined level. The external brake thereafter is energized upon occurrence of a signal representing the arrival of the needle mechanism at a predetermined location in the needle cycle.

United States Patent [191 Moran et al.

[ 1 Jan. 14, 1975 Larry M. LeBlanc, Natick, all of Mass.

[73] Assignee: Photocircuits Division of Kollmorgan Corporation, Glen Cove, NY.

[22] Filed: Apr. 3, I973 [21] Appl. No.: 347,498

[52] US. Cl. 318/375, 318/269 [51] Int. Cl H02r 3/16 [58] Field of Search 318/265, 269, 366, 367,

[56] References Cited UNITED STATES PATENTS 3,466,524 9/1969 Cooper 3l8/369 X mums l8 ERA x: 73 j Nomura ct al 318/375 X Walter 318/269 Primary Examiner-Robert K. Schaefer Assistant ExaminerW. E. Duncanson, Jr.

Attorney, Agent, or Firm-Morgan, Finnegan, Durham & Pine [57] ABSTRACT A motion control system for the direct current motor of a sewing machine wherein an external brake applies a retarding force to the motor and the sewing machine needle mechanism once the motor has reached a predetermined essentially stable level. Braking is controlled by monitoring the emf or current of the motor when it is in a dynamic braking mode and by initiating stabilized speed control of the motor when the emf or current obtains a predetermined level. The external brake thereafter is energized upon occurrence of a signal representing the arrival of the needle mechanism at a predetermined location in the needle cycle.

23 Claims, 10 Drawing Figures lyesr.

sorrom stab has CONTROL F/F 5 BRAKING 47 s R MOTION CONTROL SYSTEM FOR DIRECT CURRENT MOTORS, PARTICULARLY IN SEWING MACHINE USES BACKGROUND OF THE INVENTION Our invention relates to motion control of direct current motors of the type used in sewing machines and the like. It specifically concerns an improved motion control system wherein the parameter of motor current or motor emf is used to control the motor. such as by using auxiliary braking means to bring the motor to a predictable stop from a controlled intermediate speed level.

Control of industrial type sewing machines places stringent demands upon the machine's electrical control system. First, the sewing machine drive is subjected to continuously varying loads, and the speed of the sewing machine must be capable of continuous control by the operator, preferably being infinitely variable over the operating range. Second, it should be possible to stop the sewing machine in a relatively short period of time and in a minimum distance of needle travel under operator control. Third, it must be possible to stop the sewing machine with the needle either in the up position or in the down position. Thus, a precise positional control, and not merely random stopping, is both desirable and necessary.

Another requirement, particularly in sewing machine work, is the capability of rapid acceleration and deceleration. This requirement makes the direct current low inertia motor, directly connected to the needle mechanism. an ideal drive source, as it may be accelerated and decelerated rapidly owing to its relatively high torque and its relatively low inertia. By the same token, less braking torque is required in order to bring the motor, and the mechanism it drives, to a complete stop.

it has been the usual practice to relay upon dynamic braking of the drive motor in order to bring the needle mechanism to a halt. lf dynamic braking alone is the only source of braking, it is apparent that a specific needle position cannot be assured inasmuch as the -motor could be at any of an infinite number of intennediate speeds when braking is initiated, even should it be possible to start braking in a particular part of the mechanical cycle. in order to meet this problem, it has been the very recent practice in commerical sewing machine drives to implement a complex motor control sequencing operation wherein the motor first is slowed to a predetermined speed level and then stabilized there at least momentarily. Upon reaching this speed level, the motor subsequently is dynamically braked to a stop in response to a pulse signal that is generated when the needle mechanism, in its cycle, passes some known location.

Prior art commercial systems embodying this tech- 7 nique yield good performance when the sequencing control system for the motor is functioning properly. it has been the experience of operators, however, that these systems are much too complex, require frequent serving, are very expensive and are difficult to maintain in operation.

The complexity of these systems is attributable to several factors. First, they implement photo-responsive devices and circuits for generating pulses whose repetition rate must be reliably detected to control system operation and motor speed. While pulse train techniques do eliminate the need for tachometers and other types of speed transducers, they unnecessarily complicate the circuitry. Another complicating factor is attributable to the motor drive system itself, which operates on unidirectional current pulses of varying duty cy cles obtained from the regulated triggering of several silicon controlled rectifiers (SCRs) fed by a multiphase source. In order to obtain dynamic braking in this type of circuit, special circuitry is used to disable the SCR triggering system during dynamic braking and to reactivate the drive system to provide a low torque drive of the dc motor during the speed-stabilized mode of operation prior to the final braking step.

A further complicating factor may reside in the use of silicon controlled rectifiers for short circuiting the direct current motor during dynamic braking. A special purpose circuit must be incorporated in this case just to release dynamic braking at the end of the first braking period. This is because the SCR tends to continue conducting until the current through it is nearly extinguished.

It is an important object of the invention to overcome the problems inherent in the present day commercial systems while retaining the advantages of these systems in dc motor motion control. One principal and important advantage of our invention in that regard is the elimination of much complexity previously thought to be indispensable.

Another object of the invention is to provide a sim ple, reliable and low cost sewing machine control system which gives positive and accurate control of the motor and the sewing machine mechanism.

Another object of the invention is to join compatibly the dc motor drive signal developed from rectifying a continuously variable alternating current signal with a sewing machine motion control system using an intermediate speed regulated drive.

SUMMARY OF THE INVENTION Briefly, these and other objects of the invention are achieved through a control system in which the direct current motor is dynamically braked in response to a braking command, during which time the dynamic energy of the motor, e.g., the current or voltage generated by the motor in a braking mode, is used to develop a control signal for the system.

In a preferred embodiment, the dynamic braking current is used to detect a predetermined desired motor speed, and the motor speed thereafter is regulated by a current regulating device in series with the motor. The motor is put into the dynamic braking mode by disconnecting the normal variable direct current source from the motor terminals and connecting the motor terminals to the braking control system. Series current regulation of the motor continues until the needle mechanism reaches a predetermined position, at which time external braking means is brought into play to stop the motor from a known velocity in a prescribed time and travel distance.

in accordance with an additional feature of the inventlon, the dynamic braking current is used to develop a variable signal which, when added to a fixed voltage reference, provides a control signal for regulating the motor during the speed stabilized mode.

DETAILED DESCRIPTION OF THE DRAWINGS The following detailed description of a preferred embodiment should be referred to for a complete understanding of the invention. In the drawings:

FIG. 1 is a schematic diagram of a motor control system in accordance with the invention; and

FIG. 2 is a series of graphs, A-I, of representative waveforms and functions at various points in the system of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT NORMAL MOTOR DRIVE Referring to the left-hand portion of FIG. 1, a high torque, low inertia direct current motor 10 is excited, during normal operation, through a switch 12 from a conventional full wave bridge 14. The bridge, in turn, is excited by a continuously variable alternating current source provided by the variable transformer 15. This variable transformer includes a movable slider 16 mechanically coupled to the operator's treadle 18 of the sewing machine, as is indicated by the dashed line. The autotransformer I5 is supplied by an alternating current source from the line, e.g., 220 volts/6O Hz. applied across the transformer terminals 17a, 17b. As the operator depresses the treadle, a continuously increasing alternating current voltage appears at the movable contact 16 and therefore at the input to the full wave bridge 14. A full wave rectified signal thus drives the motor 10 in a forward direction.

A motor drive circuit of this type is disclosed in the co-pending application of Moran et al. for Sewing Machine Drive Control", Ser. No. 259,171, filed June 2, 1972 and assigned to the assignee of this invention.

The waveform applied to the motor terminals by the normal drive circuit can be seen from FIG. 2D, illustrating the qualitative nature of the motor drive signal as the operator completely releases the treadle pressure. Essentially, the motor terminal voltage includes the back emf (the dashed line) upon which is superimposed the full wave rectified voltage from the bridge 14.

BRAKING The system is put into the braking mode when the operator releases pressure on the sewing machine treadle. When the treadle returns to its normal position, the contact 16 ofthe variable transformer is shorted to the terminal 17b and thus essentially reduces to zero the applied ac signal, as already observed in FIG. 2D. Simultaneously, the movable contact 13 of the switch 12 moves from the normally closed position to make contact with the terminal 20, thereby connecting the motor to the motion control system for the sewing machine needle mechanism (the mechanism itself not being shown). With the switch 12 in this position. the motor is conditioned for both (a) dynamic braking; and (b) receiving regulated current to drive themotor at a controlled predetermined intermediate speed. Motor velocity during all operating modes is depicted in FIG. 2A, region (0) in that curve being the decelerating motor speed contour during normal drive as the treadle returns to its rest positlon.

Braking operation is automatically initiated in order to bring the sewing machine needle mechanism to a halt with the needle in the full down position, when the treadle is released. Braking occurs in three steps: (1) dynamic deceleration from existing speed to a predetermined plateau velocity (see region (I) of FIG. 2A); (2) constant speed operation at the plateau velocity until the needle mechanism arrives at a preselected location in'its cycle (region (2)); and (3) combined dynamic and external braking to bring the motor to a complete stop (region (3) of FIG. 2). In the event that the operator commands the machine to stop with the needle in the full up position, a command is given and the machine drives from the bottom stop to the top (needle up) stop automatically by accelerating to the plateau velocity and, thereafter. being braked to a complete stop by dynamic and external braking. The latter mode takes place in regions (4) and (5) of FIG. 2A.

Dynamic braking is accomplished by connecting the motor in a low impedance series current path comprised of the low impedance resistance 22 and the nominal impedance of the emitter-collector path of the switching transistor 24. During dynamic braking, the transistor 24 is turned on by an appropriate signal supplied to the transistor base via the amplifier 25. This signal is developed by the braking circuit comprised in part of the flip-flop pulse generator 27, which is totally responsive to speed-representative and positionrepresentative signals.

When motor 10 is in the dynamic braking mode, the short circuit current, i.e., the dynamic braking current, develops a voltage across the resistance 22 which is proportional to motor speed. This voltage is compared in the comparator 30 with a reference voltage applied to the amplifier 31. As the voltage across the resistance 22 drops below a predetermined value set by this reference voltage, the output of the differential comparator 30 goes low to induce a pulse (FIG. 2E) at the output of the negative edge detector 33. This latter pulse rep resents the motor's obtaining the preselected speed level and is fed through an OR gate 35 to the set (S) input of the braking control flip-flop 27, the eby driving the set (0) output high and the reset (0) output low.

The current in the dynamic braking resistor 22 is illustrated in FIG. 2C, with the corresponding position of the mode switch 12 being shown in FIG. 28. Rapid deceleration of the motor due to dynamic braking is clearly seen to occur in region (1) of the graph of FIG. 2A.

Since the braking cot t trol flip-flop 27 is normally in the reset condition, its 0 output is normally high. This condition causes an enabling signal to be applied to the amplifier 25 to turn on the transistor 24. Thus, when the treadle is released by the sewing machine operator, dynamic braking results from the short circuiting of the motor through the resistive impedance 22.

Dynamic braking continues until the voltage across the resistor 22 is equal to the set reference voltage 31, causing the pulse in FIG. 2E to fall. This negative falling edge is sensed by the detector 33 and results in a pulse (34 in FIG. 1) which sets the flip-flop 27. This sequence drives the Q output of the unit 27 high so as to enable a speed control regulating device while at the same time causing the transistor switch 24 to turn off. The 0 output, it is seen, is applied via the amplifier 38 to the base of a current regulating transistor 40, the collector of the latter being connected to an appropriate direct current source +V. At this point in the braking cycle, the circuit transfers to operation in the region (2) of FIG. 2A, wherein the motor is held at a constant, or plateau, velocity until such time as a position pulse is sensed. Speed regulation is accomplished as follows.

SPEED REGULATION Speed regulation occurs in regions 2 and 4 of FIG. 2A by supplying a regulated current to the motor through the transistor 40. In those regions the amplifier 42 is active and amplifies the voltage across the resistor 22. The gain of amplifier 42 is chosen such that the voltage applied to the anode of the zener diode 44 equals the i R (armature) voltage drop in the motor. The signal presented to the motor via the transistor 40 and the diode 45 is thus set by the zener diode and the i Ra drop in the motor.

In operation, local torque variations causing the motor speed to change are senses by the amplifier 42 and presented to the zener diode 44 as the variable component of a speed control signal. Any voltage changes in this signal component are reflected at the base of the transistor 40, which is excited with the sum of the zener diode 44 voltage and the variable i R drop voltage whenever the amplifier 38 receives an enabling signal from the Q output of the flip-flop 27. In response to signal variations at its base, the transistor 40 modifies the voltage at the diode 45 to readjust the i R drop in the motor circuit, thus keeping the motor speed constant. The motor terminal voltage is equal essentially to the sum of the motor back emf and the i R drop. Thus, motor speed is always corrected toward a constant value by varying the terminal voltage to maintain the same back emf, determined by the zener voltage, anytime i R tends to vary as a result of back emf changes.

The absolute value of the motor speed is set by the zener voltage. A small forward bias is set on the transistor 40 by the resistors 47, 48.

The transistor 40 continues to conduct until the flipflop 27 is reset. Throughout this period, therefore, the motor drives under such torque as is necessary to maintain constant speed until the needle mechanism arrives at a known position.

POSITIONAL STOP CONTROL Position pulses are generated in order to give a positional reference for the initiation of the final braking step region (3) in FIG. 2A. To this end an irregularly shaped ferromagnetic cam 50 rotating with the needle mechanism and cooperating with the reluctance pick off coil 52, generates pulses each time the lobes of the cam pass the pick off point. Those pulses are of the opposite polarities (FIG. 2F) and are applied to respective amplifiers 54, 55. The outputs of the amplifiers are gated in the AND gates 57, 58 with gating signals from the position selector flip-flop 60. The AND gate outputs are applied via the OR gate 62 to the reset (R) input of the braking control flip-flop 27.

Normally, the bottom stop (needle down) position is desired as the final needle position. That is, the Q output of the flip-flop 60 is normally high, allowing only the pulses passing to the AND gate 57 to be passed on to the braking control circuit 27. Accordingly, unless overridden by the operator, the needle will be brought to stop in the down position as a result of the flip-flop 60 normally being in the reset condition.

In summary, the braking control pulse generating unit 27, normally in the reset condition, becomes set (for normal operation) whenever motor speed drops below a pre-selected level. As long as motor speed remains above this level, the output of the voltage comparator 30 is high (FIG. 2E), and a pulse is applied to the set (S) input of the flip-flop 27 only upon extinction of the high level output of the comparator 30. The flipflop 27 output (FIG. 2G illustrating Q output thereof) then commands regulated speed operation, region (2), until such time as the needle mechanism arrives at a predetermined location. This results in the generation of a position pulse applied to the reset (R) input of the flip-flop 27, extinguishing the enabling pulse to the speed regulator and again instituting dynamic braking in region (3) of FIG. 2.

At this time, braking is supplemented by an external brake, such as the electromechanical brake represented by the amplifier 70, brake energizing solenoid 72 and the braking armature 73. Yet other types of brakes may be used satisfactorily with the invention, such as of the eddy current, magnetic particle positive mechanical detent types, etc. The braking unit 70-73 is brought into operation through detection of the trailing edge of the set (Q) output of the flip-flop 27. A trailing edge detector 75, which may be a differentiator, for example, accepts the Q output of the circuit 27; the detector output in turn triggers a one-shot pulse generator 76 to produce the waveform of FIG. 2H. During the fixed period of this pulse, the external brake is energized.

In the event that the operator wishes to select the top needle position, a command is given by moving one of the controls on the sewing machine, usually a kneeoperated lever 77. This applies a signal (FIG. 21) to the set (S) input of the needle position flip-flop 60, the top stop selector switch being shown schematically by the switch 78 in FIG. 1. This same command is applied through the amplifier 80 directly to one of the inputs of the OR gate 35 and, thence, to the set input of the flip-flop 27. This immediately sets the flip-flop and places the system in the driven mode illustrated as region (4) in FIG. 2. The motor is operated for a period of time sufficient to bring the needle mechanism to a position where it can be stopped in the full up location.

When the top stop command is applied to the flipflop 60, the condition of the Q and O outputs changes to remove the gating signal from the AND gate 57 and to enable the AND gate 58, allowing the top stop position pulses (FIG. 2H negative pulse) to pass to the OR gate 62 and thence to the R input of the flip-flop 27. The motor thus continues to drive until such time as the needle reaches the proper top location, thereupon extinguishing the pulse at the Q output the flip-flop 27 and generating a braking pulse at the output of the oneshot 76. The system then enters the external braking region (5) illustrated in FIG. 2.

From the foregoing it should be immediately appreciated that dynamic braking occurs only as long as the transistor 24 conducts as a result of an enabling signal from the braking control flip-flop. Since this signal directly controls braking, additional circuits found in systems using silicon rectifiers are not necessary. In addition, since the signal developed across the resistance 22 represents both motor speed and i R drop of the motor, it may be used both to detect plateau motor velocity and to correct speed variations from plateau velocity making unnecessary complex circuits for pulse counting and the like. All of this is an immense simplification over known commercial devices with a concommitant .saving in initial investment and maintenance time.

A further advantage of the invention is obtained as a result of the function switch 12, which removes the normal drive source from the motor during the position control sequence. This makes possible to use of the separate current regulating circuit, including the transistor 40, without having to disable in some manner the normal drive circuitry. Thus, although conceptually this approach appears to result in a duplication of circuits, it in fact results in a great simplification.

Although our development has been described with reference to a preferred embodiment, modifications and variations are possible without departing from the spirit and scope of the fundamental principles involved. For example, both positive and negative pulse logic can be used and numerous types of devices for developing position pulses and for applying external braking forces are available to those in the art and are well known. Also, the invention is not limited to any particular type of direct current motor, and any high response low inertia dc motor will provide satisfactory performance. The U 12 M4H motor available from the PMI Division of Kollmorgen Corporation, Glen Cove, New York is one such motor in this category which is satisfactory for use in the invention.

We claim:

1. A braking control system for a dc motor adapted for energization by an external power source, comprismg:

external braking means for applying a retarding force to the motor;

means responsive to the back emf of the motor when the motor is unenergized by the external power source to develop a speed representative signal; and

braking control means responsive to the speed representative signal for energizing the external braking means at a time when the motor attains a given speed level.

2. The motor braking control system of claim 1, further comprising:

means responsive to deenergization of the motor by the external power source for dynamically braking the motor to slow it to said given speed level; and

' means for driving the motor at said speed level for at least a limited period of time prior to energization of the external braking means.

3. A motion control system for a sewing machine having a needle mechanism driven by a dc motor comprising:

means for applying a dc signal to the motor to drive the needle mechanism during normal operation of the machine;

a braking circuitincluding means for connecting the motor to a series current path during braking operation thereof;

means responsive to the position of the needle mechanism for developing a position signal;

braking means for applying a retarding force to the needle mechanism; and

a brake control circuit jointly responsive to the position signal and to the dynamic braking motor current in said series current path for energizing the braking means so as to stop the needle mechanism at a predetermined position during braking operation of the system.

4. The motion control system of claim 3, wherein:

the series current path includes a low dc impedance and the brake control circuit responds in part to the voltage signal developed across said impedance.

5. The motion control system of claim 3, wherein:

the braking circuit includes a switch operable to apply the dc drive signal during normal operation and to disconnect the motor from the dc signal while connecting said motor to the series current path during braking operation.

6. The motion control system of claim 3, wherein:

the braking circuit includes switch'means connected serially in the series current path and responsive to the brake control circuit for obtaining dynamic braking of the motor during selected portions of the braking operation.

7. A motor control system for a sewing machine having a needle mechanism driven by a d.c. motor comprising:

means for applying a d.c. signal to the motor to drive the needle mechanism during normal operation of the machine;

a braking circuit including switch means for connecting the motor to a series current path during braking operation thereof for obtaining dynamic braking of the motor during selected portions of the braking operation;

, means responsive to the position of the needle mechanism for developing a position signal;

braking means for applying a retarding force to the needle mechanism;

a speed control drive circuit for applying a d.c. drive signal to the motor fora selected period of time during braking operation; and

a brake control circuit jointly responsive to the position signal and to the motor current in said series current path during braking operation for energizing the braking means so as to stop the needle mechanism at a predetermined position.

8. The motion control system of claim 7, wherein:

the speed control drive circuit is controlled by the brake control circuit so as to be operable when the motor speed obtains a predetermined level. I

9. The motion control system of claim 8, wherein:

the speed control drive circuit is operable to drive the motor with a preselected torque.

10. The motion control system of claim 9, wherein the speed control drive circuit comprises:

a regulator for applying a variable dc current signal to the motor in response to a variable control signal;

means for generating the variable control signal, said signal having a first signal component variable in accordance with motor speed variations and a second signal component which is essentially constant in value.

11. The motion control system of claim 10, wherein said generating means comprises:

means responsive to current in the motor for producing the first signal component;

reference potential means connected intermediate the speed responsive means and said regulator for developing the second signal component; and

means for applying a signal from the brake control circuit to the regulator, said signal being greater in potential than the reference potential.

12. The motion control system of claim 8, wherein: the speed control drive circuit is responsive to the position signal so as to be rendered inoperable when the motor has obtained said predetermined level and the needle mechanism, while in motion, reaches a preselected position. 13. The motion control system of claim 12, wherein: the brake control circuit energizes the braking means upon cessation of operation of the speed control drive circuit. 14. The motion control system of claim 13, wherein: the braking means is responsive to an electrical signal; and the brake control circuit generates an electrical signal having a predetermined duration so as to operate the braking means for a corresponding period of time. 15. The motion control system of claim 8, wherein: the brake control circuit comprises a pulse generator operable to produce an electrical control pulse when the motor exceeds a predetermined speed level, the braking circuit switch means being responsive to the control pulse so as to effect dynamic braking for the duration thereof. 16. The motion control system of claim 15, wherein: the position signal comprises at least one signal generated each time the needle mechanism arrives at a predetermined position in a needle motion cycle; and the brake control circuit pulse generator generates an enabling signal for the speed control drive circuit, the generator being operative to initiate the enabling pulse upon cessation of the electrical control pulse and to extinguish the enabling pulse upon the occurrence of a position pulse. 17. The motion control system of claim 16, further comprising:

means responsive to a needle position command for overriding the response of the pulse generator to motor speed so as to initiate the enabling signal for the speed control drive circuit upon such command. 18. The motion control system of claim 17, wherein: the position signal comprises at least one additional signal separated in time from the first position signal, the pulse generator responding to such other position signal so as to extinguish the enabling signal initiated by the needle command, thereby to stop the needle mechanism in a second predetermined position. 19. The dc. motor control system of claim 7, wherein:

the impedance altering means causes said low impedance to become a high impedance thereby to diminish the dynamic braking current. 20. A motor control system for a sewing machine comprising:

a direct current low inertia motor for driving the mechanism; a source of variable direct current for energizing said motor during normal operation; a direct current impedance controllable between a high impedance value and a low impedance value sufficient to dynamically brake the motor;

a controllable motor drive circuit for providing a source of direct current to run said motor at a reduced speed during selected portions during braking operation of the sewing machine;

a switch connecting the motor to said variable direct current source during normal operation and, during braking operation, for disconnecting the motor from said variable direct current source while connecting the motor to a first series current path in cluding said controllable impedance and to a second series current path including said controllable motor drive circuit; and

a control circuit responsive to the dynamic braking current in said motor during braking operation for altering said impedance from a low value to a high value when the motor speed is reduced to a desired level, and for bringing said controllable motor drive circuit into operation once said impedance is of high value.

21. The motor control system of claim 20, further comprising;

means responsive to the needle mechanism for devel oping a position signal representing the location of the needle relative to a desired position;

said control circuit being also responsive to said position signal so as to terminate operation of said drive circuit upon occurrence thereof, whereby said motor, during braking operation, is first dynamically braked and then is controllably run at reduced speed until the needle attains a predetermined location.

22. The motor control system of claim 21, further comprising;

electromechanical braking means responsive to said position signal for applying an external retarding force to the motor so as to stop the needle at a desired predetermined position.

23. A low inertia d.c. motor control system comprismeans responsive to a braking command for short circuiting the motor through a current path having a low impedance;

means for detecting the dynamic braking current in said path to develop a control signal when the magnitude of the current obtains a given value;

means responsive to the control signal for controllably altering the impedance in said current path so as to thereby control the dynamic braking current for the motor in accordance with the speed of said motor; and

speed regulating means including a current source,

a current control device in series with the motor to provide a variable current to the motor from a direct current source in response to a variable speed regulation signal during a period of time when said dynamic braking is not effective, and

means jointly responsive to the control signal and to the current through said motor for developing the speed regulation signal. 

1. A braking control system for a dc motor adapted for energization by an external power source, comprising: external braking means for applying a retarding force to the motor; means responsive to the back emf of the motor when the motor is unenergized by the external power source to develop a speed representative signal; and braking control means responsive to the speed representative signal for energizing the external braking means at a time when the motor attains a given speed level.
 2. The motor braking control system of claim 1, further comprising: means responsive to deenergization of the motor by the external power source for dynamically braking the motor to slow it to said given speed level; and means for driving the motor at said speed level for at least a limited period of time prior to energization of the external braking means.
 3. A motion control system for a sewing machine having a needle mechanism driven by a dc motor comprising: meanS for applying a dc signal to the motor to drive the needle mechanism during normal operation of the machine; a braking circuit including means for connecting the motor to a series current path during braking operation thereof; means responsive to the position of the needle mechanism for developing a position signal; braking means for applying a retarding force to the needle mechanism; and a brake control circuit jointly responsive to the position signal and to the dynamic braking motor current in said series current path for energizing the braking means so as to stop the needle mechanism at a predetermined position during braking operation of the system.
 4. The motion control system of claim 3, wherein: the series current path includes a low dc impedance and the brake control circuit responds in part to the voltage signal developed across said impedance.
 5. The motion control system of claim 3, wherein: the braking circuit includes a switch operable to apply the dc drive signal during normal operation and to disconnect the motor from the dc signal while connecting said motor to the series current path during braking operation.
 6. The motion control system of claim 3, wherein: the braking circuit includes switch means connected serially in the series current path and responsive to the brake control circuit for obtaining dynamic braking of the motor during selected portions of the braking operation.
 7. A motor control system for a sewing machine having a needle mechanism driven by a d.c. motor comprising: means for applying a d.c. signal to the motor to drive the needle mechanism during normal operation of the machine; a braking circuit including switch means for connecting the motor to a series current path during braking operation thereof for obtaining dynamic braking of the motor during selected portions of the braking operation; means responsive to the position of the needle mechanism for developing a position signal; braking means for applying a retarding force to the needle mechanism; a speed control drive circuit for applying a d.c. drive signal to the motor for a selected period of time during braking operation; and a brake control circuit jointly responsive to the position signal and to the motor current in said series current path during braking operation for energizing the braking means so as to stop the needle mechanism at a predetermined position.
 8. The motion control system of claim 7, wherein: the speed control drive circuit is controlled by the brake control circuit so as to be operable when the motor speed obtains a predetermined level.
 9. The motion control system of claim 8, wherein: the speed control drive circuit is operable to drive the motor with a preselected torque.
 10. The motion control system of claim 9, wherein the speed control drive circuit comprises: a regulator for applying a variable dc current signal to the motor in response to a variable control signal; means for generating the variable control signal, said signal having a first signal component variable in accordance with motor speed variations and a second signal component which is essentially constant in value.
 11. The motion control system of claim 10, wherein said generating means comprises: means responsive to current in the motor for producing the first signal component; reference potential means connected intermediate the speed responsive means and said regulator for developing the second signal component; and means for applying a signal from the brake control circuit to the regulator, said signal being greater in potential than the reference potential.
 12. The motion control system of claim 8, wherein: the speed control drive circuit is responsive to the position signal so as to be rendered inoperable when the motor has obtained said predetermined level and the needle mechanism, while in motion, reaches a preselected position.
 13. The motion controL system of claim 12, wherein: the brake control circuit energizes the braking means upon cessation of operation of the speed control drive circuit.
 14. The motion control system of claim 13, wherein: the braking means is responsive to an electrical signal; and the brake control circuit generates an electrical signal having a predetermined duration so as to operate the braking means for a corresponding period of time.
 15. The motion control system of claim 8, wherein: the brake control circuit comprises a pulse generator operable to produce an electrical control pulse when the motor exceeds a predetermined speed level, the braking circuit switch means being responsive to the control pulse so as to effect dynamic braking for the duration thereof.
 16. The motion control system of claim 15, wherein: the position signal comprises at least one signal generated each time the needle mechanism arrives at a predetermined position in a needle motion cycle; and the brake control circuit pulse generator generates an enabling signal for the speed control drive circuit, the generator being operative to initiate the enabling pulse upon cessation of the electrical control pulse and to extinguish the enabling pulse upon the occurrence of a position pulse.
 17. The motion control system of claim 16, further comprising: means responsive to a needle position command for overriding the response of the pulse generator to motor speed so as to initiate the enabling signal for the speed control drive circuit upon such command.
 18. The motion control system of claim 17, wherein: the position signal comprises at least one additional signal separated in time from the first position signal, the pulse generator responding to such other position signal so as to extinguish the enabling signal initiated by the needle command, thereby to stop the needle mechanism in a second predetermined position.
 19. The d.c. motor control system of claim 7, wherein: the impedance altering means causes said low impedance to become a high impedance thereby to diminish the dynamic braking current.
 20. A motor control system for a sewing machine comprising: a direct current low inertia motor for driving the mechanism; a source of variable direct current for energizing said motor during normal operation; a direct current impedance controllable between a high impedance value and a low impedance value sufficient to dynamically brake the motor; a controllable motor drive circuit for providing a source of direct current to run said motor at a reduced speed during selected portions during braking operation of the sewing machine; a switch connecting the motor to said variable direct current source during normal operation and, during braking operation, for disconnecting the motor from said variable direct current source while connecting the motor to a first series current path including said controllable impedance and to a second series current path including said controllable motor drive circuit; and a control circuit responsive to the dynamic braking current in said motor during braking operation for altering said impedance from a low value to a high value when the motor speed is reduced to a desired level, and for bringing said controllable motor drive circuit into operation once said impedance is of high value.
 21. The motor control system of claim 20, further comprising; means responsive to the needle mechanism for developing a position signal representing the location of the needle relative to a desired position; said control circuit being also responsive to said position signal so as to terminate operation of said drive circuit upon occurrence thereof, whereby said motor, during braking operation, is first dynamically braked and then is controllably run at reduced speed until the needle attains a predetermined location.
 22. The motor control system of claim 21, further comprising; electromechanical brAking means responsive to said position signal for applying an external retarding force to the motor so as to stop the needle at a desired predetermined position.
 23. A low inertia d.c. motor control system comprising: means responsive to a braking command for short circuiting the motor through a current path having a low impedance; means for detecting the dynamic braking current in said path to develop a control signal when the magnitude of the current obtains a given value; means responsive to the control signal for controllably altering the impedance in said current path so as to thereby control the dynamic braking current for the motor in accordance with the speed of said motor; and speed regulating means including a current source, a current control device in series with the motor to provide a variable current to the motor from a direct current source in response to a variable speed regulation signal during a period of time when said dynamic braking is not effective, and means jointly responsive to the control signal and to the current through said motor for developing the speed regulation signal. 